It is common practice in the field of ultrasonic imaging to make real time images of the human body by means of a linear array of transducer elements. For example, in FIG. 1 a linear array of transducer elements 8 is shown. Groups of transducer elements from transducer elements 8 are sequentially excited by electrical pulses from transmitters 1 through an element group selection unit 12 through wires 13. The result is an ultrasonic wave 14 generated in a direction along a z-axis 11, perpendicular to the linear array of transducer elements 8, as shown. When ultrasonic wave 14 is directed into a body, reflections of ultrasonic wave 14 are detected by the group of transducer elements having generated ultrasonic wave 14 or by a larger group of transducer elements from transducer elements 8. By dynamically varying the delay for each received signal, it is possible to obtain a sharply focused image in the azimuthal plane of the array, that is in the plane defined by an x-axis 9 and z-axis 11.
Ultrasound information is gathered typically for 64 to 256 discrete rays by shifting the groups of array elements used in transmission over the field of view. As shown in FIG. 2, as many as 128 transducer elements may be used to generate ultrasonic wave 14.
Reflected signals from ultrasonic wave 14 are received by transducer elements 8, transferred through wires 13, through element group selection unit 12 to a focus delay 3. Focus delay 3 varies the delay for each signal in order to obtain a sharply focussed image. The delayed signals are then summed by a Sum unit 2 and amplified as a function of depth by a depth gain control (DGC) 4 to compensate for attenuation. The signals are processed by a detector 5 and stored in a scan converter 6. The output of scan converter 6 is used to drive video monitor 7.
The detected signals at each position along x-axis 9 from ultrasound ray 14 are mapped to a corresponding ray on video monitor 7. The length of time between emission of ultrasound signals and receipt of reflections of the ultrasound signals indicates depth in the direction of z-axis 11 of an object causing the reflection. The brightness of particular portions of an image shown on monitor 7 is determined by the amount of reflected ultrasound from corresponding portions of the body Typical systems are able to generate between ten and thirty images per second essentially allowing display of an image to be a "real time" representation of scanned bodies.
The most serious drawback to the above technology is the poor focus that is obtained in the elevation plane--defined by z-axis 11 and a y-axis 10--perpendicular to the azimuthal plane of the array. The problem is often compensated for by applying a lens to the surface of linear array of transducer elements 8. This offers improvement over a limited depth of field, but, at certain depths, the resolution of the elevation plane can still be as much as ten time worse than resolution of the azimuthal plane.
Another method to improve the situation is to divide linear array of transducer elements 8 into a number of sections in the direction of y axis 10, thus producing a two-dimensional array of transducer elements. Unfortunately, each elements would need separated electrical connection and separate electronic processing up to the point at which signals are summed. Even with large scale integration, the total number of elements than can presently be processed in real time is limited to about 128. Therefore, the step to a full two-dimensional array is presently impractical.
Similarly, in the prior art, when it is desired to rotate the scan plane of a transducer, the entire array is rotated. This means that either the outer surface of the array rotates against body tissue, or there is an intermediate fluid layer and a separate acoustic window. Both approaches involve rotary seals. The latter approach may result in degraded image quality.